OLEDs using an organic substance are promising for applications to inexpensive, large area, full-color, solid-state emissive displays or writing light source arrays and have been under active studies. OLEDs generally have a light emitting layer sandwiched between a pair of opposing electrodes. With an electric field applied to the electrodes, electrons and positive holes are injected from the cathode and the anode, respectively, into the light emitting layer, where they are recombined. Whereupon the electrons fall from the conduction band to the valence band, emitting light as an energy difference between the bands, which is observed as luminescence.
Conventional OLEDs have low luminescence efficiency and low durability, i.e., a short service life. Various techniques providing solutions to these problems have recently been proposed.
For example, JP-A-2003-68466 discloses a light emitting diode comprising a substrate having formed thereon an anode, a cathode, and a light emitting layer intermediate between the electrodes. The light emitting layer contains a host material and a dopant incorporated in the host material. Aiming to improve luminescence efficiency and life, the dopant comprises a light emitting material and a non-light-emitting material. However, the system using BCP (bathocuproine) recited in JP-A-2003-68466 is unsatisfactory in life because of the high ionization potential of BCP (6.1 eV), still leaving room for improvement.
JP-A-2003-77674 discloses a light emitting diode, the light emitting layer of which contains (1) a host material having electron transport and/or hole transport properties, (2) a first compound that produces phosphorescence at room temperature, and (3) a second compound that produces phosphorescence or fluorescence at room temperature and has a longer maximum emission wavelength than the first compound. The second compound is allowed to emit light at high efficiency. More specifically, the second compound is either a phosphorescent compound that does not emit light alone at high efficiency or a fluorescent compound that has various emission colors but with not so high emission efficiency as a phosphorescent compound at any wavelength. The disclosure teaches that the first compound showing phosphorescence at room temperature, when combined with the second compound, serves as a sensitizer to enhance the luminescence of the second compound. However, the proposed light emitting diode is insufficient in life, and an improvement is still awaited.